Aviation phosphate ester functional fluids with enhanced acid scavenging properties

ABSTRACT

The present invention is an aviation phosphate ester functional fluid of enhanced acid scavenging properties comprising a phosphate ester base oil and a mixture of aliphatic epoxide and carbodiimide acid scavengers.

This application claims the benefit of U.S. Ser. No. 60/700,700 filed Jul. 19, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to aviation phosphate ester functional fluids resistant to acid build-up.

2. Related Art

Phosphate ester based functional fluids are well known in the lubrication art and have been utilized for years as electronic coolants, power transmission and hydraulic fluids and refrigeration equipment fluids to mention a few. Hydraulic fluids which are to be used in aircraft applications must meet certain performance criteria among which are thermal stability, fire resistance, low susceptibility to viscosity changes over a wide range of temperatures, hydrolytic stability and good lubricity.

In current available commercial hydraulic fluids, phosphate esters are the most commonly used base stocks of which tributyl phosphate, isopropylated triphenyl phosphates, n-butyl diphenyl phosphate, and di-n-butyl phenyl phosphate are widely used components. Phosphate ester based functional fluids useful as aircraft hydraulic fluids have been described in U.S. Pat. Nos. 5,464,551, 3,723,320 and 3,679,587.

The commercial aircraft system designers have opted to use phosphate esters because of the superior fire resistance properties of phosphate esters. However, the reduced fire risk is accompanied by a debit in various performance parameters such as lesser stability when compared to the hydrocarbon based fluids or silicate ester fluids also in use.

Phosphate esters tend to absorb atmospheric moisture readily and build up high concentrations of water (0.3 to 1% water, sometimes more). Like all esters, these fluids have the potential to hydrolyze (react with water to form alcohols and acids). The presence of high concentrations of water typically results in extent of hydrolysis setting the life of the fluid (point at which the fluid has to be replaced). The OEMs Acid Number specification limit for in-service fluids has been set at 1.5 mg KOH/g.

Particular mixtures of phosphate esters have been identified as being useful as the base fluid for phosphate ester based functional fluids, especially those used in aircraft. Thus references can be made to Skydrol(® LD-4 which reportedly contains 30-35 wt % di-butyl phenyl phosphate, 50 to 60 wt % tri-butyl phosphate, 5 to 10 wt% viscosity index improver, 0.13 to 1 wt % diphenyldithio-ethane copper corrosion inhibitor, 0.005 to 1 wt % perfluoroalkylsulfonic acid salt anti-erosion agent, 4 to 8 wt % acid scavenger and about 1 wt % 2,6 di tertiary butyl p-cresol as an antioxidant.

As aircraft hydraulic systems have advanced and operating conditions have become more severe, special phosphate ester base stocks have been described. U.S. Pat. No 5,464,551 describes a base stock comprising between about 10 to 100 wt % of a trialkyl phosphate, between about 0 and 70 wt % dialkyl aryl phosphate and from about 0 to 25 wt % alkyl diaryl phosphate, the sum of the proportionate amount of each component equally 100%. Specifically, it is stated that the alkyl substitutent of the trialkyl phosphate contains between 3 and 8 carbons, preferably between 4 and 8 carbons, more preferably between 4 and 5 carbons and are bonded to the phosphate moiety via a primary carbon. Preferably the alkyl substitutents of the tri alkyl phosphate, the di alkyl aryl phosphate and the alkyl di aryl phosphate are isoalkyl groups, substantially iso C₄ and iso C₅ alkyl groups.

Another advance in base fluid is described in WO 96/17517, published Jun. 13, 1996, which teaches a phosphate ester base stock comprising 60 to 95 wt % of a tri alkyl phosphate and about 5 to 40 wt % of a second component selected from the group consisting of triaryl phosphate, a mixture of triaryl phosphate and linear polyoxy alkylene material, and a linear polyoxyl alkylene material, which base stock is free of di alkyl aryl phosphate and alkyl di aryl phosphate. In such base fluid the alkyl groups are, aliphatic and alicyclic group wherein the aliphates group are straight or branched chain, and contain independently, 1 to 12 carbon atoms and di aryl groups are, independently, phenyl or alkyl substituted phenyl having from 7 to 20 carbon atoms.

Phosphate ester functional fluids are formulated with an acid scavenger to prevent acid build-up. U.S. Pat. Nos. 3,723,320, 3,941,708 and 5,464,551 disclose epoxide acid scavengers for use in phosphate ester based functional fluids and teach that the preferred epoxides are 3,4-epoxycycloalkyl carboxylates.

The use of alkyl epoxides including the 3,4-epoxycycloalkyl carboxylate acid scavengers currently used in commercial phosphate ester based functional fluid compositions result in fluids which are susceptible to the formation of carboxylic acids (weak acids) during use in an aircraft. During service in an aircraft, the acid number of the phosphate ester based functional fluid can reach 1.6 mg KOH/g despite the presence of the epoxide acid scavenger in the fluid. The acidity is explained by the hydrolysis of the carboxylate group of the epoxide acid scavenger that lead to a weak carboxylic acid and an alcohol. This reaction is illustrated below:

where R is an alkyl group containing from 2 to 12 carbon atoms.

Carbodiimides are well known acid scavengers. Dr. Achim Fessenbecker, Rhein Chemie Rheinau GmbH (NLGI, vol. 61, 3, p. 10, 1996) disclosed that the Total Acid Number (TAN) of isopropylphenyl phosphate containing a carbodiimide was reduced in a coke bottle test as compared with the pure base fluid and thus carbodiimide was deemed capable of neutralizing strong acids as phosphoric acids.

U.S. Pat. No. 6,235,687 relates to lubricating oils exhibiting anti-rust properties that are obtained by adding to the lubricating oil an acidic anti-rust additive and an acid scavenger such as carbodiimide, in a specific sequence.

U.S. Pat. No. 5,614,483 relates to lubricant base materials containing ester groups that may be stabilized by adding a small quantity of carbodiimides and the service life lubricants manufactured therefrom may be decisively extended.

U.S. Pat. No 6,143,702 discloses lubricating oils of enhanced oxidation stability that are obtained by adding to the lubricating oil a mixture comprising n-phenyl-1-naphthyl amine and an acid scavenger such as carbodiimide.

DESCRIPTION OF THE INVENTION

The present invention is directed to a phosphate ester based functional fluid comprising a major amount of a organic phosphate ester base oil and a minor additive amount of a combination of alkyl epoxy and carbodiimide acid scavengers.

The functional fluids of the present invention include a major amount of an organic phosphate ester base stock.

Phosphate ester base fluids are generally of the formula:

wherein each R is the same or different and selected from alkyl or alkoxy alkyl group having 2 to 20 carbon atoms, aryl groups (e.g., phenyl) and substituted aryl groups containing up to 20 carbon atoms where the substituents are alkyl, phenyl, alkyl phenyl or phenyl alkyl and wherein the alkyl group contain form 1 to 10 carbons.

Typical organo phosphate ester base stocks include esters selected from trialkyl phosphates, triaryl phosphates, dialkyl aryl phosphates, alkyl diaryl phosphates, and alkylated triaryl phosphates that contain from 3 to 8, preferably from 3 to 6, more preferably 3 to 5 carbon atoms in the alkyl groups and mixtures thereof. Examples of the foregoing esters include tri-n-butyl phosphate, tri-isobutyl phosphate, tri-sec-butyl phosphate, di-isobutyl pentyl phosphate, tri-2-ethyl hexyl phosphate, n-butyl di-isobutyl phosphate, di-isobutyl n-butyl phosphate, n-butyl diphenyl phosphate, isobutyl diphenyl phosphate, di-isobutyl triphenyl phosphate, isopropylated triphenyl phosphates and butylated triphenyl phosphates. Preferably, the trialkyl phosphate esters are those of tri-n-butyl phosphate, tri-isobutyl phosphate and mixtures thereof.

The alkyl substituted phenyl groups of the triaryl phosphates include phenyl groups having from 1 to 3 alkyl substituents wherein the alkyl groups are straight a branched chain groups of form 1 to 14 carbon atoms and further, wherein each alkyl substituted phenyl group has a maximum of up to 20 carbons. Examples of triaryl phosphate esters include, by way of example tri(isopropyl phenyl)phosphate, tri(tert-butylated phenyl)phosphate, tri cresyl phosphate, and the like and mixtures thereof.

Combinations of tri alkyl phosphates and tri aryl phosphates include a mixture of tributyl phosphate and tri(isopropylphenyl)phosphate at about a 6:1 to 7:1 ratio, and a mixture of tri butyl phosphate and tri(butylated phenyl)phosphate at about a 6:1 to 7:1 ratio.

The organo phosphate ester base stock useful in the present invention, therefore comprises about 10 to 100 wt % of a trialkyl phosphate, about 0 to 75 wt % of a dialkyl aryl phosphate, about 0 to 30 wt % of an alkyl diaryl phosphate and about 0 to 15 wt % of a triaryl phosphate. Preferably the alkyl groups of the esters contain 3 to 5 carbons, more preferably 3 to 4 carbons.

The phosphate ester based fluid typically comprises a fire resistant phosphate ester base oil and a viscosity index improver, an acid scavenger and an erosion inhibitor. The fluid may also contain one or more rust inhibitors, one or more antioxidants, one or more metal corrosion inhibitor.

It has been discovered that the epoxide acid scavengers currently used in such functional fluids are not effective in neutralizing the carboxylic acids produced in the in-service fluid. It also has been discovered that the carbodiimides are very effective in neutralizing the carboxylic acids but not effective in neutralizing strong acids (alkyl phosphoric acids) produced by the hydrolysis of the phosphate ester based functional fluids. The combination of an epoxide and carbodiimide acid scavengers, however, provide a combination to reduce the acidity produced by the alkylated phosphoric acids and carboxylic acids in the phosphate ester based functional fluids during use in environments containing water, e.g., in aircraft hydraulic systems.

The functional fluids of the present invention contain an anti-erosion additive to inhibit flow-induced electrochemical corrosion.

Suitable erosion inhibitors are disclosed, for example, in U.S. Pat. Nos. 5,464,551 and 3,679,587, the entire disclosures of which are incorporated herein by reference in their entirety. Preferred erosion inhibitors include the alkali metal salts, and preferably the potassium salt, of a perfluoroalkyl or perfluoro-cycloalkyl sulfonate as disclosed in U.S. Pat. No. 3,679,587. Such perfluoroalkyl and perfluorocycloalkyl sulfonates preferably encompass alkyl groups of from 1 to 10 carbon atoms and cycloalkyl groups of from 3 to 10 carbon atoms.

Suitable anti-erosion additives are alkali metal salts of perfluorooctyl sulfonic acids such as FC-98 or FC-95, from, for example, 3M, Minneapolis, Minn.

The erosion inhibitor is employed in an amount effective to inhibit erosion in the power transmission mechanisms of an aircraft and, preferably, is employed in an amount of from about 0.01 to about 0.5 wt %, based on the total weight of the hydraulic fluid composition and more preferably from about 0.02 to about 0.1 wt %. Mixtures of such anti-erosion agents can be used.

In addition to containing the major amount of a phosphate ester base stock and the small amount of the erosion inhibitor, the functional fluids of the present invention also include from about 4 wt % to about 20 wt % of auxiliary additives as previously indicated selected from the group consisting of anti-oxidants, acid scavengers, viscosity index (VI) improvers, rust inhibitors, defoamers and metal corrosion inhibitors.

The hydraulic fluid compositions of this invention can further optionally comprise an antioxidant or mixture of antioxidants in an amount effective to inhibit oxidation of the hydraulic fluid or any of its components.

Useful antioxidants include trialkylphenols, polyphenols, phenyl alpha naphthyl amines (PANA) and di(alkylphenyl) amines.

Representative antioxidants include, by way of example, phenolic antioxidants, such as 2,6-di-tert-buty-p-cresol, tetrakis[methylene(3,5-di-tert- butyl-4-hydroxyhydrocinnamate)]-methane (Irganox®1010 from Ciba Geigy), bis(3,5 di-tert-butyl-4 hydroxyphenyl)methane(Ethanox 702 from Ethyl corporation), 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert butyl-4-hydroxyphenyl) benzene (Ethanox 330 from Ethyl Corporation) and the like; amine antioxidants including, by way of example, diarylamines, such as di(noctylphenyl)amine (Vanlube ® 81), phenyl-α-naphthylamine, (noctylphenyl)naphthylamine (Irganox L06), alkylphenyl-α-naphthylamine, or the reaction product of N-phenylbenzylamine with 2,4,4-trimethylpentene (Irganox® L-57 from Ciba Geigy), diphenylamine, ditoylamine, phenyl tolyamine, 4,4′-diaminodiphenyl-amine, di-p-methoxydiphenylamine, or 4-cyclohexylaminodiphenylamine. Still other suitable antioxidants include aminophenols such as N-butylaminophenol, N-methyl-N-amylaminophenol and N-isooctyl-p-aminophenol as well as mixtures of any such antioxidants.

A preferred mixture of antioxidants comprises 2,6-di-tert-butyl-p-cresol and di(octylphenyl)amine (e.g., a 1:1 mixture). Another preferred mixture of antioxidants is 2,6-di-tert-butyl-p-cresol, di(octylphenyl)amine and 6-methyl-2,4-bis(octylthio)-methyl]-phenol (e.g., 1:2:4 mixture). Still another preferred mixture of antioxidants is 2,6-di-tert-butyl-p-cresol, di(octylphenyl)amine and tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)]methane (e.g., a 1:2:3 mixture).

The antioxidant or mixture of antioxidants is employed in an amount effective to inhibit oxidation of the hydraulic fluid. The antioxidant or mixture of antioxidants is employed in an amount ranging from about 0 to about 3 wt %, more preferably from about 0.5 to 2.5 wt % and still more preferably at from about 1 to 2 wt % based on the total weight of the hydraulic fluid composition.

Viscosity Index Improver (VI) additives useful in hydraulic fluid compositions of this invention include polyacrylate esters and poly (alkyl methacrylate) esters of the type described in U.S. Pat. Nos. 5,817,606 and 3,718,596, the disclosures of which are incorporated herein by reference in their entirety.

Typically, the viscosity index improver is of high molecular weight, having a number average molecular weight between about 50,000 and about 100,000 and a weight average molecular weight between about 200,000 and 350,000. Viscosity index improvers are commercially available from Rohm and Haas Company and referred to as poly (alkyl methacrylate) PA-7570, PA 6703, PA 6744 and PA 6961-PMN. The hydraulic fluid compositions of this invention can contain from about 3 wt % to about 10 wt % of the viscosity index improver (or an active ingredient basis), preferably about 4 to about 6 wt % (on an active ingredient basis) based on the total weight of the functional fluid composition. The VI improver can be mixed with a portion of the phosphate ester base stock, typically as a 1:1 mixture and then added to the balance of the functional fluid.

Rust inhibitors suitable in this invention include metal corrosion inhibitors such as benzotriole derivatives and dihydroimidazole derivatives such as Amine-O commercially available from Ciba-Geigy and Vanlube RI-G commercially available from Vanderbilt. Other suitable rust inhibitors include those described in U.S. Pat. Nos. 5,035,084, 4,206,067 and 5,464,551 the entire disclosures of which are incorporated herein by reference in their entirely. Additional non-limiting examples include calcium dinonylnaphthalene sulfonate, a Group I or Group II metal overbased and/or sulfurized phenate, sulfonate or carboxylate, a compound of the formula: R⁴N[CH₂CH(R⁵)OH]₂ wherein R⁴ is selected from the group consisting of alkyl of from 1 to 40 carbon atoms, —COOR⁶ and —CH₂CH₂N[CH₂CH(R5)OH]₂ where R⁶ is alkyl of from 1 to 40 carbon atoms, and each R⁵ is independently selected from the group con-sisting of hydrogen and methyl, including N,N,N′,N′-tetrakis(2-hydroxypropyl) ethylene diamine and N,N-bis(2-hydroxyethyl) tallowamine (e.g., N tallow amine alkyl-2,2′-iminobisethanol, sold under the trade name Ethomeen T 12), and diphenyl dithioethane (sold under the tradename FH-132).

The Group I and Group II metal overbased and/or sulfurized phenates, preferably are either sulfurized Group I or Group II metal phenates (without CO₂ added) having a Total Base Number (TBN) of from greater than 0 to about 200 or a Group I or Group II metal overbased sulfurized phenate having a TBN of from 75 to 400 prepared by the addition of carbon dioxide during the preparation of the phenate. More preferably, the metal phenate is a potassium or calcium phenate. The phenate advantageously modifies the pH to provide enhanced hydrolytic stability.

Each of these components are either commercially available or can be prepared by art recognized methods. For example, Group II metal overbased sulfurized phenates are commercially available from Chevron Chemical Company, San Ramon, Calif. under the trade name OLOA® including, OLOA 219®, OLOA 216Q® and the like and are described by Campbell, U.S. Pat. No. 5,318,710, and by MacKinnon, U.S. Pat. No. 4,206,067. Likewise, N,N,N′,N′ -tetrakis (2-hydroxy-propyl)ethylenediamine is disclosed by MacKinnon, U.S. Pat. No 4,324,674. The disclosures of each of these patents are incorporated herein by reference in their entirety.

Group I or II metal dinonylnaphthalene sulfonates, such as calcium dinonylnaphthalene sulfonate (Na-Sul 729 commercially available from King industries) may also be used as a rust inhibitor in the hydraulic fluid composition in an amount ranging from 0.2 to 1.0 wt % of the hydraulic fluid composition.

The rust inhibitor or mixture of rust inhibitors is employed in an amount effective to inhibit the formation of rust. The rust inhibitor is employed in an amount ranging from about 0 to about 1 wt %, preferably 0.001 to about 1 wt %, more preferably about 0.005 to about 0.5 wt %, and still more preferably at about 0.01 to 0.1 wt % based on the total weight of the hydraulic fluid composition. In a preferred embodiment, the rust inhibitor comprises a mixture of N,N,N′,N′-tetrakis(2-hydroxypropyl)ethylenediamine and a Group II metal overbased phenate (e.g., a 5:1 mixture). In another preferred embodiment, the rust inhibitor comprises a mixture of N,N-bis(2-hydroxyethyl)tallowamine (Ethomeen® T 12) and a Group II metal overbased phenate (e.g., a 5:1 mixture).

The functional fluid compositions of this invention further comprise an acid control additive, acid receptor, or acid scavenger (all three terms used as synonyms in the patent and public literature) in an amount sufficient to neutralize acids formed in such functional fluids, especially functional fluids utilized as aircraft hydraulic fluids, such as phosphoric acid and its partial esters. Suitable acid control additives are described, for example, in U.S. Pat. Nos. 5,464,551, 3,723,320 and 4,206,067, the disclosures of which are incorporated herein in their entirety.

Preferred acid control additives have the formula:

where R is selected from the group consisting of an alkyl group of from 1 to 10 carbon atoms, substituted alkyl of from 1 to 10 carbon atoms and containing from 1 to 4 ether oxygen atoms therein and cycloalkyl of from 3 to 10 carbon atoms, each R′ is independently selected from the group consisting of hydrogen, alkyl of from 1 to 10 carbon atoms and —C(O)OR″ where R″ is alkyl of from 1 to 10 carbon atoms optionally containing from 1 to 4 ether oxygen atoms, therein or cycloalkyl of from 3 to 10 carbon atoms.

Preferred acid scavengers of the above formula are the monoepoxide 2-ethylhexyl 7-oxabicyclo[4.1.0]heptane-3-carboxylic acid, which is disclosed in U.S. Pat. No. 3,723,320, or mono epoxide 2-(ethoxy ethoxy)ethyl7+L-OXABICYCLO [4.1.0]heptane-3-carboxylate or mixtures thereof, and monoepoxide 7-oxa-bicyclo [4.1.0]-heptane-3,4-dicarboxylic acid, dialkyl esters (e.g., the di-isobutyl ester). Dialkyl esters of this monoepoxide are also disclosed in U.S. Pat. No. 3,723,320. The trialkyl and tetraalkyl esters are prepared via conventional Diels-Alder reaction procedures via a suitable unsaturated trialkyl or tetraalkyl ester and a suitable 1,3-diene. The Diels-Alder reaction provides for 4 +2 cyclo addition to provide for a cyclohexene derivative having the suitable trialkyl or tetraalkyl esters. The unsaturation in the cyclohexane is utilized to provide for epoxide formation via conventional methods.

Suitable unsaturated trialkyl and tetraalkyl esters are known in the art. For example, tetraethyl ethylene tetracarboxylate is available from Fluka (Ronkonoma, N.Y.). The alkyl groups of this tetraethyl ester can readily be exchanged via conventional techniques to provide for other esters as defined above.

The use of such di-, tri- and tetraalkyl esters of this monoepoxide provide for enhanced seal compatibility for the formulation of this invention as well as with conventional formulations employing conventional trihydrocarbyl phosphate basestocks with the ethylene propylene seals used in aircraft hydraulic systems.

The swelling of ethylene propylene seals in an aircraft hydraulic system due to contact of said seals with an aircraft hydraulic fluid composition containing 7-oxabicyclo-[4.1.0]heptane-3-carboxylic acid, 2-ethylhexyl ester as the acid scavenger is further reduced by: (a) replacing at least a portion of the monoepoxide 2-ethylhexyl 7-oxabicyclo-[4.1.0] heptane-3-carboxylic acid, acid scavenger with an acid scavenger of the formula:

where R is selected from the group consisting of an alkyl group of from 1 to 10 carbon atoms optionally containing from 1 to 4 ether oxygen atoms therein, and cycloalkyl of from 3 to 10 carbon atoms, each R′ is independently selected from the group consisting of hydrogen, alkyl of from 1 to 10 carbons atoms and —C(O)OR where R is as defined above, with the proviso that at least one of R′ is —C(O)OR.

Preferably, at least 20%, and more preferably from about 20% to about 100% of the 7-oxabicyclo-[4.1.0]heptane-3-carboxylic acid, 2-ethyhexyl ester acid scavenger is replaced by the di ester acid scavenger.

The acid control additive whether as mono- di-, tri- or tetra-ester is employed in an amount effective to scavenge the acid generated, typically as partial esters of phosphoric acid, during use of the functional fluid, especially during use as an aircraft hydraulic fluid. Preferably, the acid control additive is employed in an amount ranging from about 4 to about 10 wt %, based on the total weight of the hydraulic fluid composition, and more preferably from 5 to 8 wt % and still more preferably from 6 to 8 wt %, based on the total weight of the fluid.

Carbodiimides suitable in this invention might be mono carbodiimides or poly carbodiimides. Useful carbodiimides are illustrated by the following structural formula R¹—(N ═C═N)_(n)—R² wherein R¹ and R² are the same or different and are hydrogen, hydrocarbyl groups, preferably hydrocarbyl group containing 1 to 20 carbons, more preferably 1 to 18 carbons, or nitrogen and/or oxygen containing hydrocarbyl groups and n is 1 to 5, preferably 1 or 2. Thus R¹ and R² can be C₁ -C₁₂ aliphatic groups, C₆-C₁₈ aromatic groups or aromatic-aliphatic groups. Thus R¹ and R² may be, for example, hydrogen, alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, pentyl, 2-methylbutyl, hexyl, heptyl, octyl, 2-ethylhexyl, nonyl, decyl, undecyl, dodecyl and the like, alkenyl groups such as propenyl, butenyl, isobutenyl, pentenyl, 2-ethylhexenyl, octenyl and the like, cycloalkyl groups such as cyclopentyl, cyclohexyl, methyl-cyclopentyl, ethylcyclopentyl and the like, aryl groups such as alkyl substituted phenyl groups for example toluyl, isopropylphenyl, diisopropylphenyl and the like, aralkyl groups such as benzyl, phenetyl and the like. Carbodiimides wherein n is 1 are monocarbodiimides exemplified by the following:

di-isopropyl carbodiimide, di-n-butyl carbodiimide, methyl-tert-butyl carbodiimide, diphenyl carbodiimide, di-p-tolyl carbodiimide, 2,2′-diethyl-diphenyl carbodiimide, 2,2 ′-diethoxy-diphenyl carbodiimide, 2,2 ′, 6,6′-tetraethyldiphenyl carbodiimide and the like. The preferred carbodiimide is the 2,2′,6,6′-tetraisopropyldiphenyl carbodiimide such as Additin® RC8500 commercially available from Rheine Chemie.

Carbodiimides wherein n is greater than 1 are polycarbodiimides exemplified by the following:

tetramethylene-ω,ω′-bis-(tert-butyl carbodiimide), hexamethylene-ωω′-bis- (tert-butyl carbodiimide), tetramethylene-ω,ω′-bis-(phenyl carbodiimide) and those compounds which may be obtained by heating aromatic polyiso-cyanates such as 1,3-di-isopropyl-phenylene-2,4-isocyanate, 1-methyl-3,5-diethyl-phenylene-2,4-diisocyanate and 3,5,3′,5′-tetra-isopropyl-diphenyl-methane-4,4-di-isocyanate in the presence of tertiary amines, basically reacting metal compounds, carboxylic acid metal salts or non-basic organometal compounds at a temperature of at least 120° C., according to the process disclosed in German Patent 1,156,401.

Typical amounts of carbodiimide additive that can be used in the functional fluid compositions of this invention are from about 0.05 to about 5.0 wt %, based on total weight of fluid, preferably from 0.5 to 3 wt % based on total weight of fluid.

The invention is further described by reference of the following comparative examples and non-limiting examples.

In the Examples, the epoxide employed was 2-ethylhexyl-7-oxabicyclo [4.1.0]heptane-3-carboxylic acid. Acid number was determined by ASTM D-974.

EXAMPLE 1

In this Example, 10 gm of a used phosphate ester based hydraulic fluid (Fluid 1) that contained 1.73% of residual epoxide acid scavenger was stirred at 40° C. for 72 hours with 0.1 gm (0.276 mmole) carbodiimide RC 8500 (Fluid 2), and with 0.1 gm (0.387 mmole) epoxide acid scavenger (Fluid 3). The Fluid 1 had an initial acid number of 1.2 mg KOH/g attributable to the presence of carboxylic acids. Fluid 2 of this invention containing the carbodiimide gave a 63.7% acid number reduction whereas Fluid 3 with additional epoxide acid scavenger gave 27.1 % acid number reduction, showing that the mere addition of more epoxide is not sufficient to control the acidity attributable to weak carboxylic acids in the fluids. TABLE 1 Component Fluid 1 Fluid 2 Fluid 3 Base oil Tributyl phosphate, wt % 78.7086 77.7086 78.7086 Triaryl phosphates, wt % 11.80 11.80 11.80 Additives VI Improver Erosion Inhibitor Rust Inhibitors {close oversize brace} wt % 7.7614 7.7614 7.7614 Antioxidants Defoamer Purple Dye Epoxide Acid Scavenger, wt % 1.73 1.73 2.73 Carbodiimide, wt % (RC 8500) 0 1.0 0 Properties Acid Number, mg KOH/g 1.2 0.436 0.875 Reduction % — 63.7 27.1

EXAMPLE2

This Example demonstrates that the carbodiimide RC 8500 is very effective in reducing the acid number of a petroleum fraction HAGO (Fluid 4) containing naphthenic acids (carboxylic acids) whereas the epoxide acid scavenger is not. Two 100 ml samples of Fluid 4 were heated at 80° C. for 6 hours with 1 gm of each of the acid scavengers (Fluids 5 and 6) and the Acid Number determined. The epoxide acid scavenger did not provide any acidity reduction attributable to the weak carboxylic acids whereas the carbodiimide gave 83.5% reduction of the acidity attributable to weak carboxylic acids. TABLE 2 Fluid 4 Fluid 5 Fluid 6 HAGO Base oil — 6 hrs @ 80° C. 6 hrs @ 80° C. Additive Carbodiimide, wt % — 1.1 0 Epoxide Acid Scavenger, — 0 1.1 wt % Properties Acid Number 1.21 0.20 1.21 Reduction, % 0 83.5 0

EXAMPLE 3

This Example illustrates that the presence of up to 8.0 wt % carbodiimide (RC 8500) in the phosphate ester based functional fluid (Fluid 7) did not degrade the viscometric properties. Moreover, no additive drop out was observed after one week storage at —65° F. The combination of as little as 1 wt% of the carbodiimide with epoxide acid scavengers (Fluid 9) also did not degraded key fluid the properties yet reduced the TAN to a level below that achieved using either the epoxide or the carbodiimide (at a higher concentration) alone. TABLE 3 Component Fluid 7 Fluid 8 Fluid 9 Target Limits Base Oil Tributyl phosphate 72.18 74.48 74.18 Triaryl phosphates, wt % 12.10 12.10 12.10 Additives VI Improver Antioxidants Rust Inhibitors {close oversize brace} wt % 7.72 7.72 7.72 Erosion Inhibitor Defoamer Epoxide, wt % 0 5.70 5.0 Carbodiimide (RC-8500), wt % 8.0 0 1.0 Properties Specific Gravity, 25° C./25° C. 0.9964 0.9973 0.9971 0.970-1.020 Viscosity @ −65° F., cSt 1752 1273 1324  2000 max Viscosity @ −65° F., cSt⁽¹⁾ 1706 1261 1305  2000 max Viscosity @ 100° F., cSt 11.17 10.19 10.40  9.0-12.5 Acid Number, mg KOH/g 0.05 0.04 0.03  0.10 max ⁽¹⁾After 1 week at −65° F. All three samples were Clear & Bright.

EXAMPLE 4

This Example demonstrates that whereas in Example 2 it is shown that carbodiimides are good acid scavengers for weak carboxylic acids, the carbodiimides such as RC 8500 are poor acid scavengers for the alkyl phosphoric acids produced from the hydrolysis of the phosphate esters. About 15 mL of the fluid was placed in sealed glass ampoules with a piece of copper and steel wires to catalyze the hydrolysis. The fluid was spiked with water to make total water content of 0.5 wt %. The ampoules were placed in an oven at 150° C. The Acid Number was determined as a function of time. The test is completed when the Acid Number of the fluid has reached 1.5 mg KOH/g (fluid life). The results showed that Fluid 7 containing 8.0 wt % carbodiimide reached a 3.08 mg KOH/g after only 48 hours whereas Fluid 8 containing only the epoxide acid scavenger went to 244 hours before reaching an Acid Number of 1.59 mg KOH/g. The combination of 1 wt % carbodiimide and 5.0 wt % epoxide (Fluid 9) performed well and reached an Acid Number of 1.88 mg KOH/g after 222 hours. It is important to note that Fluid 9 contained 0.7 wt % less epoxide than Fluid 8. TABLE 4 Fluid 7 Fluid 8 Fluid 9 Time, Acid Time, Acid Time, Acid hours Number hours Number hours Number 0 0.05 0 0.04 0 0.03 48 3.08 48 0.02 48 0.01 96 0.02 96 0.03 144 0.04 144 0.04 192 0.06 192 0.06 222 0.24 222 1.88 240 0.40 240 4.83 244 1.59

EXAMPLE 5

A fresh portion of a phosphate ester hydraulic fluid which did not contain any acid scavengers was spiked with cekanoic acid to produce a test fluid 10, the acid number of which was measured as 1.5.

This test fluid 10 was then treated with epoxide alone, carbodiimide along and with a mixture of epoxide and carbodiimide. The fluids (Fluid 11, Fluid 12, Fluid 13 and Fluid 14) are presented in Table 5 and the data after 72 hours show that the mixture of the carbodiimide and epoxide (Fluid 14) reduces the acid number to a greater extent then just the epoxide alone at 5.70 g epoxide dose (Fluid 11) or carbodiimide alone at 1.0 g carbodiimide dose (Fluid 12). TABLE 5 Acidity Component Reduction Fluid 10 Fresh phosphate 467.50 gm Ester hydraulic fluid 1.50 gm Cekanoic acid 2.50 gm water Acid # (ASTM D-974) 1.5 mgKOH/g Fluid 11 Fluid 10 94.30 wt % Epoxide 5.70 wt % Acid # after 72 hrs (ASTM D-974) 1.169 mgKOH/g 22.1% Fluid 13 Fluid 10 92.00 wt % RC 8500 8.00 wt % Acid # after 72 hrs (ASTM D-974) 0.190 mgKOH/g 87.3% Fluid 14 Fluid 10 94.00 wt % RC 8500 1.00 wt % Epoxide 5.00 wt % Acid # after 72 hrs (ASTM D-974) 0.903 mgKOH/g 40.0% Fluid 12 Fluid 10 99.00 wt % RC 8500 1.00 wt % Acid # after 72 hrs (ASTM D-974) 1.18 mgKOH/g 22.1% 

1. A functional fluid comprising: a) a major amount of an organo phosphate ester base stock; b) from about 0.05 to 5 wt % based on the total weight of the fluid of a carbodiimide having the following structural formula R¹—(N ═C ═N)_(n)—R² wherein R₁ and R₂ are the same or different and are hydrogen, hydrocarbyl groups or nitrogen and/or oxygen containing hydrocarbyl groups and n is 1 to 5; c) from about 4 to 10 wt% based on the total weight of the fluid of an epoxide acid scavenger having the following structural formula

wherein R is selected from the group consisting of alkyl having from 1 to 10 carbon atoms, substituted alkyl of from 1 to 10 carbon atoms and containing from 1 to 4 ether oxygen atoms; each R′ is independently selected from the group consisting of hydrogen, alkyl from 1 to 10 carbon atoms and C(O)—OR″ where R″ is selected from the group consisting of alkyl of from 1 to 10 carbon atoms optionally containing from 1 to 4 ether oxygen atoms, and cycloalkyl of from 3 to 10 carbon atoms.
 2. The functional fluid of claim 1 wherein the organo phosphate ester base stock comprises: i) from about 10 to 100% of a trialkyl phosphate; ii) from 0 wt % to about 75 wt % of a dialkyl aryl phosphate; iii) from 0 wt % to about 30 wt % of an alkyl diaryl phosphate; and iv) from 0 wt% to about 15 wt% of a triaryl phosphate.
 3. The functional fluid of claim 2 wherein the alkyl groups of the esters have 4 to 5 carbon atoms. 